Prior to October 9, 2003, the Moss Motors part number 121-400 O-Ring specified for use with the MGA 1600 and early MGB with banjo type rear axle would not fit any MG rear axle application. That part was too large diameter and too thick. Those parts from inventory have been scrapped.

The part previously stocked under part number 121-405 is a perfect fit for the 1600 and early MGB rear hub, and has been re-specified for use with those models, and has been moved into the 121-400 bin. This same O-ring will work with the 1500 hubs with just a slight stretch at installation.

It is my understanding that the 1500 part number 121-405 in the catalog will either be changed to 121-400 or will be noted to use 121-400 in its place. Any parts in stock under 121-405 part number may be re-labeled 121-400 and may be used in any 1500 or 1600 or early MGB rear hub.

For clarification of which hub is which, refer to:
http://mgaguru.com/mgtech/rearaxle/ra103.htm
This shows pictures of the different rear hubs, and dimemsions of the O-rings and the gland grooves.